Loop antenna shield



April 22, 1947. R. H.'BRYAN ET'AI. 2,419,480

LOOP ANTENNA SHIELD Filed May 18, 1944 ll wc( 1,6" f7 221411.@@11/ if:fa/1 145 /mz/ Y y Wvw 5MM Q Patented Apr. 22, 1947 LOOP ANTENNA SHIELDRobert H. Bryan, Oak Park, and Truman L.

Johnston, Wilmette, Ill., assgnors to Stewart- Warner Corporation,Chicago, Ill., a corporation of Virginia Application May 18, 1944,Serial No. 536,116

(Cl. Z50-33) 7 Claims.

The present invention relates in general to Aa loop antenna shield, andmore particularly to an electrostatic shield for such antennas.

One of the objects of the present invention is to provide an improvedantenna shield of tear 'drop or streamlined design to reduce theaerodynamic drag when used on airships or airplanes. Another object isto provide an electrostatic shield that will shield the loop antennaelectrostatically in any position without producing unde` magnetic eldwhich is used to determine the direction of the incoming signal is ableto pass through to the loop antenna very readily.

Another object is to provide improved means for shielding the loopantenna so as to reduce so called precipitation static.

A further object is to provide a simple and eiective method ofconstructing an electrostatic shielding shell.

Still another object is to provide a simple method of providingshielding conductors on the streamlined supporting shell.

A stil1 further object is to provide an electrostatic shield on a loopantenna in which there is no appreciable loss of sensitivity as a resultof the shielding.

There are other objects of the invention which, together with theforegoing, will be described more in detail in the specification that isto follow, with reference to the accompanying drawings illustratingcertain specific embodiments thereof.

It is well known that when a loop antenna is employed inaerial-navigation for direction finding, it is desirable for the loop tobe shielded in order to eliminate the pickup of non-directionalelectrostatic fields. Suchv shielding will insure that the null in thedirective electromagnetic eld Y being employed is definite and certain.However, if such shielding is placed about the loop in position torotate with it, undesirable aerodynamic drag occurs.

The suggestion has therefore been made to employ a streamlined shell orcasing of fixed position about the loop antenna. However, in suchinstances almost invariably the shielding is not uniform with respect tovarying angular positions of the loop and the nulls in the directivesignal being received vary and are uncertain.

Precipitation static is the name given to extraneous electrical chargeswhich have caused considerable difficulty in the radio receiving sets ofairplanes, and became much more troublesome with the advent of highspeed ying. It was thought to be caused by the accumulation ofelectrical charges from rain, mist, and snow, and more recently has beenassumed to be a charge caused by the friction of the wind on thesurfaces of the airplane. It is, however, very evident that a staticcharge of considerable magnitude is built up and sporadicallydischarged, causing great difficulties of radio reception in theoperation of the radio direction nders.

Shielding the loop antenna reduces the elect of precipitation staticvery materially and in conjunction with precipitation static dischargedevices eliminates it to the point where a radio compass can function.The present inventionv combines a loop shield and a means fordischarging precipitation static. The invention employs two sets ofparallel conductors each open at one end with their other ends connectedto ground, providing a so called Faraday method of shielding thatpermits the electromagnetic Waves to penetrate therethrough butsubstantially stops electrostatic penetration. By the employment of twosets of open ended parallel conductors the electromagnetic ei'ect fromthe electrostatic currents discharging to ground cancel one another. Thetear drop design of the casing or shell supporting the conductorspermits the free rotation of the loop antenna Within, as well aslocating the grounding bars for the parallel conductors as far away fromthe loop as possible. These conductors, being located on the outside ofthe plastic shell, discharge the precipitation static before it canaccumulate.

In practicing the invention, it will thus be seen that a plastic shellof streamlined or tear drop design is molded of sufficient size so as toenclose the loop antenna and permit its free rotation. Parallel groovesare molded in the outer surface of the plastic shell which isconstructed from a dielectric plastic such as laminated Bakelite. Alongitudinally extending groove is formed in each end of the plasticshell. Separate grounding conductors are mounted in each longitudinalgroove and connect to an annular metallic ring molded in the base of theshell. Each longitudinal conductor is connected to conductors lyingparallel to one another in the grooves about the shell, one half beingconnected to one longitudinal conductor and the other half to the other.The parallel conductors are open ended. The shielding shell is securedto the metal surface of the airplane in such position that its axisparallels the line of ilight of the plane, thereby effectively shieldingand streamlining the loop antenna.

The drawings, comprising Figs. 1 to 4, inclusive, show two specificembodiments of the invention.

Referring now to the drawings:

Fig. 1 is a perspective elevation of the invention with a certainportion of the plastic shell cut away to show the loop antenna;

Fig, 2 is an end elevation of the same, illustrating how the parallelconductors are secured to the grounding longitudinal conductor on oneside and open ended on the other;

Fig. 3 is a sectional view of Fig, 1, taken along the line 3 3 thereof,looking in the direction of the arrows; and

Fig. 4 is a side elevation of a modified form of the inventionillustrating a little different type of streamlining wherein the shellis reduced in diameter at the bottom to provide a pedestal.

In the drawings, like reference characters apply to similar partsthroughout.

Referring now to the drawings, the reference character 6 indicates ashell of a dielectric plastic of general streamline shape of sufficientsize to enclose a loop antenna l rotatably mounted therein. The shell orcasing S may be formed from laminated Bakelite and pressed into theshape shown, or may be molded. Into each side of the shell ll, parallelgrooves 8 are formed and extend into a channel 9 at one end of ltheshell and terminate short of a channel iii, while on the other side ofthe shell 6 similar grooves join the channel l and terminate short ofthe channel 9.

Metallic conductors ll and E2 are secured in the channels 9 and I0.respectively, at each end of the shell 6 and are connected to chargecollecting conductors ifi and i5, respectively, secured in parallelgrooves 8 lying in each hemisphere of the shell. The longitudinalconductors l I and I2 are suitably connected with the metallic ring Il'extending about the base of the shell 8, and suitably secured thereto bymolding or in any other convenient manner.

A number of openings IB are provided about the bottom periphery of theshell 6 and permit the fastening of the same to the surface i9 of .theairplane by means of rivets or screws 28. This brings the annular ringIl into direct electrical connection with the metal surface of theplane, thus grounding it,

rIhe parallel conductors lil and l may be made from any good conductingwire, such as copper or silver, and may be soldered to the longitudinalconnecting wires H and i2 respectively. The longitudinal connectingwires ll and i2 extend to the openings 2i and Z2, and are soldered tothe annular conducting ring il.

A very convenient method of constructing this parallel conductorarrangement is to employ a metallic spray such as copper or silver,whereby the metal will lie in both the horizontal and longitudinalgrooves formed in the plastic shell and the excess metal is wiped orscraped away.

The loop antenna 7 may be of any usual and well known construction andis freely rotatable within the shell S by any convenient means. Theshell 5 is secured to the surface of the plane in such direction thatits blunt nose faces the direction of travel of the plane, thus reducingaerodynamic drag to its minimum.

It will be seen that the parallel conductors are so placed about theenclosing plastic shell that the conductors in each hemisphere aregrounded at one end and open at the other. As before pointed out, thistype of construction not only forms a very eifective electrostaticshield of the Faraday type for the loop antenna l, but neutralizes anycurrents in the conductors. As a result, the conductors do not form aclosed circuit and the electromagnetic iield produced as theelectrostatic charges iiow to ground is therefore negligible. Theelectromagnetic field from the directive wave can. however, freelypenetrate the structure.

Since precipitation static is mainly electrostatic in character, theimproved construction is very effective in eliminating it, theelectromagnetic fields around the parallel conductors substantiallycanceling each other. The conductors are spaced sufficiently from theloop antenna that any uncanceled fields will have no appreciable effectupon the loop.

Use of parallel conductors in this manner about the two sides of theinsulating shell 6 also prevents any appreciable efect on the nulls inthe directive signal for any position of the loop antenna since freepassage of the electromagnetic wave is provided,

The construction of the modified form of the invention shown in Fig. 4is essentially the same as that of l, except that the lower portion ofthe streamlined shell 2E has a reduced diameter of streamline crosssection shown at 25, furnishing a base or pedestal. In this case thehorizontal grooves containing the parallel conductors it and i5 do notneed to extend below the level of the loop antenna 'i shown in thedotted lines.

The longitudinal conductors Il and IZ may be connected to an annularmetallic ring at the bottom of the pedestal the same as il, Fig. 3,which furnishes `the method of grounding the shield to the airplanestructure. The shielding conductors may be grounded in any othersuitable manner,

The construction of Fig. l furnishes a more convenient mounting attimes, though the operation of the two forms or shapes of shields areessentially the same.

From the foregoing, it will appear that the shield is suiiicientlystreamlined to add but little drag, and this slight drag is constant,irrespective of the position of the loop antenna. Since the groundedcollector conductors are at opposite ends of the shield, the arrangementof the shielding wires is symmetrical, and fields induced by currentilow through the wires of one group are substantially canceled by theelds due to current ilow through conductors of the other group. Becauseall of the circuits are open, that is, none of the conductors completelyencircles the shield, these conductors cannot produce fields ofsubstantial strength and as a result the loop antenna will be eective todetect the direction of the incoming directional signal with a highdegree of precision. The charges due to precipitation static are rapidlydissipated to the skin and frame of the airplane since the conductorsare located on the streamlined shell and preferably on the outside ofthe shell, and thus the harmful interfering eifects of precipitationstatic are substantially completely eliminated,

While we have shown and described particular embodiments of ourinvention, it will be apparent that numerous variations andmodifications thereof may be made without departing from the underlyingprinciples of the invention. We therefore desire, by the followingclaims, to include within the scope of our invention all such variationsand modications by which substantially the results of our invention maybe obtained through the use of substantially the same or equivalentmeans.

We claim:

1. An electrostatic shield for a loop antenna comprising, a shell ofdielectric material, a plurality of spaced parallel conductors mountedaround said shell, each extending slightly less than one-half thedistance around the shell, and a pair of collecting conductorsconnected` to ground and located at opposite ends of the shell, one halfof said parallel conductors being connected to one collecting conductorand th'e other half of said parallel conductors being connected to theother collecting conductor.

2. An electrostatic shield for a loop antenna comprising, a shell ofdielectric material, a plurality of spaced parallel conductors mountedaround said shell and substantially enclosing it, each of saidconductors extending slightly less than one-half the distance around theshell, a pair of collecting conductors connected to ground and locatedat opposite ends of the shell, one half of said parallel conductors eachhaving one end connected to one of said collecting conductors, theopposite end being free, and the other half of said parallel conductorseach having one end connected to the other of said collecting conductorswith their opposite ends free.

3. An electrostatic shield for a loop antenna comprising a shell ofdielectric material having a streamline shape, a plurality of parallelconductors on said shell, a grounded metal ring secured to said shell, apair of longitudinal conductors each connected t0 said ring and locatedat diametrically opposed ends of said shell, one half of said parallelconductors being connected at one end to one of the longitudinalconductors and the other half of said parallel conductors beingconnected at one end of the other longitudinal conductor.

4. An electrostatic shield for a loop antenna comprising, a, shell ofdielectric material having a streamline shape, a plurality of spacedparallel conductors on said shell, a grounded metal part secured to saidshell, a pair of longitudinal conductors at the opposite ends of saidshell and each connected to said part, one half of said parallelconductors being connected at one end to one of the longitudinalconductors and the other half of said parallel conductors beingconnected at one end to the other longitudinal conductor, the other endsof both sets of parallel conductors being open.

5. An electrostatic shield for a loop antenna comprising a shell ofdielectric material having a streamline shape,` a grounded metal partsecured to the base of said shell, a pair of longitudinal conductorseach connected to said metal part and located 4on the shellsubstantially 180 apart, a plurality of parallel'oonductors mountedaround said shell, one half of said parallel con ductors being connectedat one end to one of the longitudinal conductors and the other half ofsaid parallel conductors being connected at one end to the otherlongitudinal conductor, the other ends of both sets of parallelconductors being open, and means for clamping said shell to the frame ofan airplane.

6. An electrostatic shield for a loop antenna, comprising a shell ofdielectric material having a streamline shape and having a base formedfor attachment for the skin of an airplane, a plurality of groups ofparallel conductors extending partly around said shell, a metal partsecured to said shell at the base thereof so as to make groundingcontact with the skin of an airplane to which the shell is secured, aplurality of collecting conductors each connected to said part, saidcollecting conductors being spaced substantially 180 apart, the parallelconductors of each group being connected at one end only to one of saidcollecting conductors.

7. A stationary electrostatic shield for a loop antenna, comprising astreamlined dielectric housing in which the loop antenna may rotate,

n said housing having a plurality of sets of parallel grooves therein,the grooves of each set having their adjacent ends joined by a grooveextending transversely thereof, the transverse grooves being spacedapart substantially 180, metallic conductors substantially iilling allof said grooves, and means for grounding the conductors in thetransverse grooves.

ROBERT H. BRYAN.

TRUMAN L. JOHNSTON.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 644,530 Mercer Feb. 27, 19002,297,488 Luderitz Sept. 29, 1942 1,824,579 Threlkeld et al. Sept. 22,1931 2,234,998 Worrall Mar. 18, 1941 2,343,306 Lear Mar. 7, 1944 D.122,616 Lear Sept. 17, 1940 D. 132,380 Page Sept, 1'?, 1940

